1. Memory color effects of natural objects on color constancy
Thorsten Hansen,
Maria Olkkonen, Sebastian Walter and Karl R. Gegenfurtner
Department of Psychology
University of Giessen
Germany

2. Color constancy
Color constancy is the constant perception of object color (reflectance) under changing illumination.

4. Mechanisms of color constancy
Local image contrasts: cone ratios across edges are largely illumination invariant
Global image color: assuming a “gray world” can discount many illumination changes
Memory color: the characteristic color of familiar objects can be recalled from long-term memory
Earlier studies found a positive effect of memory color, but it is unclear how large the effect was and whether it would also work with natural objects

5. Stimuli: Fruits and vegetables

6. Paradigm: Achromatic setting
The subjects had to adjust the images until the isolated objects appeared neutral gray
Moreover, the subjects had to adjust the images to the typical object color of the fruit or vegetable
In a control experiment,
achromatic settings were
made with homogeneous
discs and pink noise
Settings were made in a color chamber. Illumination was calibrated to match the monitor background color.
N = 17 observers

7. DKL color space
Derrington Krauskopf Lennie
Cone-opponent color space: two chromatic axes: L-M, S-(L+M) and a luminance axis: L+M.
S-(L+M) 90
L-M
180
270

8. Chromatic adjustment
Subjects adjusted the chromatic distribution in the isoluminant plane of DKL space
The chromatic distribution was rotated and scaled towards the new location

9. banana space

10. Results: banana
typical color
achromatic setting

11. Results: more fruits On average, the achromatic setting is made
away from the gray point
in a direction opposite to the natural color

12. Control condition with synthetic stimuli
No significant shift from the neutral gray point in the control condition.

13. Quantifying the effect
lemon
grapes
orange
carrot
zucchini
lettuce
banana
The memory color effect was quantified by relating the excursion of the gray setting to the typical setting
The index was on average 10%
… and significantly different from zero for all natural stimuli

14. Evidence from hue scaling Hansen & Gegenfurtner (2006), Visual Neurosci., in press.

15. Summary
Images of fruits or vegetables are perceived in the characteristic color of the object if all pixels in the image are gray
For the objects to appear gray, subjects adjusted the average color 7–16% in the color direction opposite to the memory color. For example, the banana that appears gray was adjusted to a bluish color
No such effect was found for the homogeneous discs or the pink noise stimuli
Memory colors can support color constancy

16. Summary
is gray
looks gray

17. Conclusions
The perceived color of natural objects is biased towards their natural color.

18. Results: individual observers

19. Calculation of memory color index
Calculate the projection of the grey setting to the typical setting
Divide the amplitude of the projection by the amplitude of the typical setting

20. Experimental chamber
Monitor
Neon lamps
Neon lamps
Observer